1. Field of the Invention
The disclosure generally relates to the production of hydrocarbons from subsea formations. More particularly, the disclosure relates to the risers and related support structures used in such production.
2. Description of the Related Art
In producing hydrocarbons from subsea formations, a number of wells are typically drilled into the sea floor in positions that are not directly below or substantially within the outline of a floating offshore structure, such as a floating offshore production platform. The produced hydrocarbons are subsequently exported via subsea pipelines or other means. Current engineering practice links the offset wells with the offshore structure through risers that have a catenary curve between the structure and the sea floor. Wave motion, water currents, and wind cause movement of the floating offshore structure and/or risers themselves with corresponding flex and stress in the risers. The current state of the art has accommodated the flex in the risers by incorporating flexible joints at suitable locations between pipe segments in the riser. However, the flexible joints are more expensive and less reliable than pipe segments that are welded together.
Steel Catenary Risers (SCRs) are designed to be coupled to the floating offshore structure through pull tubes extending from the lower keel of the offshore structure to the upper part of the offshore structure. A pull tube is generally a long conduit that forms a guide through which the SCR is pulled from the seafloor and coupled to the offshore structure. The pull tube is attached to the offshore structure at an angle from the vertical so as to be in line with a natural angle that the installed SCR would assume on a calm day. As the offshore structure shifts laterally and vertically, the pull tube helps reduce stresses on the SCR. However, the pull tube itself is then stressed and can fail with time. The pull tube is attached to the offshore structure at different points along the length of the pull tube and thus flexes about its attachment points to the offshore structure as the SCR flexes and bends from the movement of the floating offshore structure. A first attachment point can be located a distance from the lower end of the pull tube. A second attachment point for the pull tube to the offshore structure can be at a distance further upward from the first attachment point to allow additional flexibility in the pull tube. Further, the pull tube can be provided with a bending stiffness that varies from the first attachment point to the lower end of the pull tube. Typically, a tapered stress joint is placed along the pull tube adjacent one of the attachment points and is sized to control the SCR stress.
There are two types of stress joints that have been used in the past. The first one is an assembly of pipe segments welded together. The pipe segments typically have a progressively smaller wall thickness for each segment of a given inside diameter that results in a tapered assembly of the segments with the thinnest segment distal from the middle of the welded assembly to allow more flexibility at the end of the assembly for the SCR. Such assemblies typically are challenged by fatigue performance at the welds between the segments for the many years in which the SCR will likely be used. The second type of stress joint is a forged tapered stress joint. The forging accomplishes a similar goal as the first type by progressively thinning the wall thickness toward the end of the forging typically in the length of 40 ft. However, due to the desired length of a pull tube stress joint, additional pull tube segments are typically welded to the forging. Thus, the challenge is still fatigue performance at the welds between the segments and forging. Another challenge can be cost and manufacturing schedules specific to a lengthy forging piece.
More particularly, FIG. 1 is an exemplary prior art schematic of a pull tube stress joint. The pull tube stress joint 50 is adapted to allow a riser 53 to be pulled therethrough and includes a tapered middle section 51, which can be one of the two types described above of a progressively smaller wall thickness of an assembly of pipe segments or a continuous forging. The middle section 51 has a length “L”, which can for example be about 40 feet (12 meters) and is typically centrally disposed relative to a pivot point “A”, so that a ½ L length extends 20 feet (6 meters) outward therefrom in this example. A pull tube joint 52 is welded to the end of the middle section 51 at welding B about 20 feet (6 meters) from the pivot point A. The stresses at welding B are such that special and expensive welding procedures known as a “C Class Girth Weld” are typically specified to attempt to reduce fatigue at the welding B at the 20 foot (6 meter) location from the pivot point A. While a longer middle section could be used to extend the ½ L length from the pivot point A, the expense and timing of production and handling make such an option unsuitable for practical reasons.
There remains then a need for an improved design of a suitably long lasting, cost effective pull tube stress joint for catenary risers.